Powdery mildew

Powdery mildew is a fungal diseased caused by a number of different fungal organisms. This is a common disease in curcurbits including cucumbers, pumpkins, squash, cantaloupe and melon. Some powdery mildew strains also affect other vegetables, fruits, ornamentals or even turf, but are less common.

Symptoms:

• Leaves get a white, powdery coating in a splotchy pattern. 
• Leaves may turn yellow or brown and eventually fall off.
• In extreme cases, the powdery coating can appear on the fruit.

Disease progression:

• The disease usually occurs later in the growing season on curcurbits, usually when temperatures are around 20°C - 27°C and humidity is low.
• The disease is spread by wind-bourne spores and is encouraged by low light conditions and poor air circulation, such as plants too close together or having a lot of weeds nearby.
• Most powdery mildew fungi over-winter in infected plant debris.
• Powdery mildews do not require water to germinate. Water can kill fungi and inhibit germination. Spores and fungi are sensitive to extreme heat and direct sunlight.
• Too much nitrogen fertilizer causes lush tender growth and dense foliage which encourages the disease. 
• Powdery mildew is an obligate parasite which means that it needs living tissue in order to grow.
• Heavily infested powdery mildew plants will eventually die. 
• Infected fruit will have off-flavours and will not store as long as healthy fruit.

Control:

• Grow plants in full sun.
• Whenever possible, grow cultivars that are resistant to powdery mildew. (see below)
• Rotate your cucurbit crops within your garden, preferably a 3-year rotation.
• Remove weeds around the plants.
• Stake plants if possible to encourage good air flow.
• Mulch your soil to avoid spreading powdery mildew through soil splash.
• Do not over-apply nitrogen fertilizer.
• Thoroughly spray your plant leaves with plain water early in the day. Washing off the spores has been shown to reduce the severity of infection.
• There is some evidence that milk, baking soda and garlic sprays can reduce the severity of powdery mildew, but these remedies do not completely eradicate it. 
• Prune leaves if infection is minor. Generally, more severe infections happen late in the season. Harvest fruits as soon as possible and remove plant debris which can be hot-composted.

For a list of disease-resistant cultivars see: Cornell University disease resistant curcubit cultivars

Sources:

Powdery Mildew on Vegetables. (n.d.). Retrieved March 22, 2021, from http://ipm.ucanr.edu/PMG/PESTNOTES/pn7406.html

Shamuyarira, M., Tembo, L., & Mhungu, S. M. (2016). Effectiveness of organic substances in the control of powdery mildew (Sphaerotheca fuligenia) of butternut (Cucurbita moschata PEPO). International Journal of Agronomy and Agricultural Research (IJAAR), 9(1), 92-99.